Aside from the changes I just made, none of my commits in the last few months to this plugin would have had an effect on performance. They were style and translation changes.
So if there has been a noticeable deterioration in performance in those very large topics in the last few days, it is probably something core related.
I can take a look in more detail at what might have caused this in core Discourse later this week, but you should raise this with the Discourse team.
*edit @parisa You may want to have a read of this topic: Performance improvements on long topics?. Another host with a topic of a similar size (i.e. 4.5K posts) has the same issue: